cherniy_chack, thanks for those points! I see yours have a different or completely removed engine lifting eye than mine which also means, judging from your attached photo, that you could reach the first bolt with a regular 13mm spanner without it restricting you. Mine goes higher up and is in the way for tightening that first manifold bolt.
No diesel leaks as far as I can tell, all the fluid that leaks out pitch black, thicker than diesel and most importantly doesn't smell the way diesel does. The engine is completely stock as it's been owned by a fire department and serviced regularly completely according to the book.
The other "jets" as you call them or pots as referred to earlier in this thread are completely clean and dry, the only one that appear to be leaking is the first one. Good point regarding the joint of the intake manifold and the air/throttle body. I'll check those three bolts as well, to make sure those are tight.
And as you write, I'm pretty certain that the intake manifold has never been removed on this engine (as noted above owned by the fire department and the only other owner before me continued the precise servicing without being a mechanic himself, so he hasn't altered the engine and the service book is filled up according to recommendations) so if it's the intake manifold gasket then it has gone bad or loosened overtime. Engine has 124.000km on it, so it's not used much either.
I'll go ahead and remove that engine lifting eye that's currently in the way and then check the first bolt of the manifold to be sure, along with the three that hold the throttle body to the intake manifold as you pointed out! Hopefully this will help and there will be no need for new gaskets or shim. If not I'll go ahead to look at replacing them when spring and warmer weather arrives!